Memory

ObjectivesIn this chapter, you will:

-Learn the differences between the three key types of physical memory: ROM, DRAM and SRAM.

-Be able to identify and explain the differences between memory module types: SIMMs, DIMMs and RIMMs

-Understand how to install memory into a PC

-Identify and understand Parity and error correcting memory

-Troubleshoot tricky memory problems

-Understand the PC’s logical memory layout

Memory

Memory is the work space for the computer’s processor.

It is a temporary storage area where the programs and data being operated on by the processor must reside.

Memory can be viewed as a collection of sequential locations, each with a unique address capable of storing information of a certain size.

Memory is often called RAM, for Random Access Memory because you can randomly (as opposed to sequentially) access any location in memory.

Memory is used by writing data to an address, or reading data from an address.

ROM(Read-Only Memory)

Characteristics: ROM chips contain vital, permanently stored

information written to them by the manufacturer. ROM is nonvolatile in that the information remains

stored regardless of whether an electrical current is present.

ROM is non-changeable in that it can’t be written over, only read from.

This is why it’s called read-only chip. Information on a ROM chip never changes. If an error occurs on the chip, it must be replaced.

ROM Chip

RAM (Random Access Memory)

Sometimes referred to as main memory or main system memory.

All PCs must have RAM to store the results of processing and data being created and modified.

Considered volatile (new info may be sent to RAM in its place or when electrical current is removed).

CPU uses this RAM to temporarily store programs and data.

Speed of RAM is measured in nanoseconds (billionths of a second), where disk speeds is measured in milliseconds (thousandths of a second).

RAM module

RAM Chips

RAM can refer to both the physical chips that make up the memory and the logical mapping and layout of that memory.

Logical mapping and layout refer to how the memory addresses are mapped to actual chips and what address locations contain which types of system information.

Physically, the main memory in a system is a collection of chips or modules containing chips that are usually plugged into the motherboard.

These chips or modules vary in their electrical and physical designs and must be compatible with the system to function properly.

RAM chips

More RAM installed on a motherboard, more storage room there is for applications and data.

Increase in available memory can provide better performance, up to a point.

RAM Module Bank

DRAM(Dynamic RAM)

Referred to simply as “RAM” or “main memory” in the context of PC systems.

When you hear “how much memory” a computer has, response usually refers to DRAM.

The main advantage of DRAM are that it is very dense, meaning you can pack a lot of bits into a very small chip, and it is inexpensive.

The memory cells in a DRAM chip are tiny capacitors that retain a charge to indicate a bit. The problem with DRAM is that it is Dynamic.

DRAM chips

DRAM(Dynamic RAM)

It can hold a charge (data) only for a short time, so it must be refreshed periodically. If not data is lost.

Refreshing the memory unfortunately takes processor time away from other tasks because each refresh cycle takes several CPU cycles to completed.

Some systems allow you to alter the refresh timing parameters via the CMOS Setup.

DRAM is used in PC systems because it is inexpensive and the chips can be densely packed, so a lot of memory capacity can fit in a small space. Unfortunately, DRAM is also slow.

DRAM chips

SRAM(Static RAM)

Static RAM is so named because it does not need the periodic refresh rates like DRAM. Because of SRAM’s design, it is much faster than DRAM and fully capable of keeping pace with modern processors.

Extremely fast (as fast as 7-9 nanoseconds and 2 or less nanoseconds for ultrafast SRAM).

SRAM’s design calls for a cluster of six transistors for each bit of storage. The use of transistors means that refresh rates are not necessary.

Compared to DRAM, SRAM is much faster but also much lower in density and much more expensive.

SRAM Chip

SRAM(Static RAM)

The lower density means that SRAM chips are physically larger and store many fewer bits overall.

cache memory resides on SRAM.Also used for CMOS configuration setups and requires a

small amount of electricity provided by a backup battery on the system board to keep its data.

Some SRAM comes in burst-mode format (containing its own clock), making it compatible with the PowerPC processors.

Used on credit-card memory cards (available in 128K, 256K, 512K, 1MB, 2MB and 4MB sizes) with battery lives of 10 yrs or more.

L2 COAST SRAM module

SDRAM(Synchronous DRAM)

More recent technology, uses a clock to synchronize a memory chip’s input and output signal.

This clock is coordinated with the CPU’s clock so the timing between the Memory Bus and Module are synchronized.

With this performance is enhanced by using SDRAM to save time in executing commands and transmitting data.

This is also used on video cards to move some of the graphics processing out of the main CPU.

SDRAM Module

DDR SDRAM(Double Data Rate SDRAM)

DDR SDRAM memory is an evolutionary design of standard SDRAM in which data is transferred twice as quickly.

DDR memory achieves the doubling in performance by transferring twice per transfer cycle: once at the leading edge and once at the trailing edge of the cycle.

DDR SDRAM is almost identical to previous SDRAM, except for the double clocking.

Module

RDRAM(Rambus DRAM)

RDRAM is a fairly radical memory design found in high end PC systems.

Rambus developed what is essentially a chip to chip memory bus, with specialized devices that communicate at very high rates of speed.

Each RDRAM on a RIMM essentially operates as a standalone module sitting on the 16 bit data bus.

Internally, each RDRAM chip has core that operates on a 128 bit wide bus split into eight 16 bit banks running at 100MHz.

Other improvements include separate control and data signals on the memory bus.

Module

Memory Modules

Originally, system had memory installed via individual chips. They are often referred to as dual inline package chips.

DIP chips are a time consuming and labor intensive way to deal with memory.

To eliminate these problems, manufactures soldered the chip onto a daughter board.

Most modern systems have adopted the single inline memory module (SIMM), dual inline memory module (DIMM), or Rambus inline memory module.

DIP(Dual Inline Package)

DRAM chip was common when memory was soldered directly onto the system board.

The pins install into holes that extend into the surface of the circuit board.

Dual Inline Package

SIPP(Single Inline Pin Package)

Originally, DRAM chips were soldered onto daughter boards called SIPPs.

The pins install into holes that extend into the surface of the circuit board.

Single Inline Pin Package

SIMM(Single Inline Memory Module)

DRAM chips are grouped together into SIMMs.SIMMs are a series of chips referred to as a module.Mixing different types of SIMMs within the same memory bank prevents CPU from accurately detecting how much memory it has.

72 pin SIMM

30 pin SIMM

64 pin SIMM

SIMM Cont.(Single Inline Memory Module)

However, one can substitute a SIMM with a different speed (60ns or 70ns) within the same memory bank, only if the replacement is equal to or faster than the replaced module.All memory taken together will be set to the speed of the slowest SIMM.Are available in two physical types –30 pin (9 bit) and 72 pin (32 and 36 bit)Macintosh SIMMs are 64 pin

DIMM(Dual Inline Memory Modules)

DIMMs usually hold standard SDRAM and DDR SDRAM chips, they are distinguished by two different physical characteristics. SDRAM DIMMs have 168 pins , one notch on each side, and two notches along the contact area.DDR SDRAM DIMMs have 184 pins, two notches on each side and one notch along the contact area.All DIMMs are either 64 bit (nonparity) or 72 bit (parity or ECC) wide data paths.

184 pin DDR DIMM

168 pin DIMM w/ECC

RIMM(Rambus Inline Memory Modules)

Much like DIMMs, RIMMs are also double sided.RIMMs are available with 184 pins, one notch on either side, and two notches centrally located in the contact area.RIMMs are either 16 bit (non ECC) or 18 bit (ECC) wide data paths.RIMM modules contain 4, 8, 16 or more RDRAM chips are wired in series.Any RIMM sockets without a RIMM installed must have a continuity module. This enables the memory bus to remain continuous from the controller through the module until the bus finally terminates on the motherboard.

186 pin RIMM

Memory DiagnosticsMemory integrity is one of the first things tested by the POST (Power On Self-Test) or called “cold boot.”Parity: the state of either oddness or evenness assigned to a given byte of data.Parity Checking: Is a way a computer uses a special set of logical rules and chips to make decisions based on the parity (state) of a particular byte.Parity circuit is what tells the computer how to remember what it sent, in order to compare what it reads.

Odd/Even Parity

If a given byte has an even number of 1s (even parity), the parity bit is turned on.

If that byte has an odd number of 1s (odd parity), the parity bit is turned off.

When a CPU calls on memory for the data, the data is intercepted by the parity circuit before it gets to the CPU.

Fake or Disabled Parity

Parity chip that sends a 1 or 0 to the parity circuit to supply parity on the basis of which parity state is expected.

Regardless of whether the parity is valid, the computer is fooled into thinking that everything is valid.

More common way to reduce cost SIMMs is to simply disable the parity completely, or build a computer without any parity checking capability installed.

ECC(Error Correction Code)

Uses a special algorithm (mathematical logic), works with the memory controller to add error correction code bits to each data bit when they’re sent to memory.When data requested, memory controller decodes the error correction bits and determines the validity of their attached data bit.ECC is capable of detecting and correcting one-bit error.Some can detect the unusual 2,3,4-bit memory errors.ECC was used mainly in high-end, expensive computers.

EDO(Extended Data Output)

Data is stored in a memory address as an electrical charge.The time it takes between the discharge of electricity from a particular location, until that location is capable of holding a new charge is called a wait state.EDO memory allows a memory address to hold a piece of data for multiple reads.EDO memory doesn’t discharge a memory address until a new bit of data is written to that particular location.EDO memory is faster than previous types of memory because it doesn’t produce as many wait states.

Logical Memory LayoutThe following sections give you an understanding of

the PC hardware memory layout, which is consistent no matter which operating system you use.

The only things that change are how your operating system uses these areas and how they are managed by the OS.

The kinds of memory are:Conventional (base) memoryUpper memory area (UMA)High memory area (HMA)Extended memory (XMS)Expanded memory (obsolete)

Conventional (base) MemoryThe original PC/XT type systems was designed to

use 1MB of RAM.The first 384KB was used by the operating systems

leaving the remaining 640KB for running programs.

This area is often termed the 640KB memory barrier.

This barrier largely affects 16bit software, such as DOS and WIN 3.1 and is much less a factor with 32bit software and operating systems.

Upper Memory AreaThe term Upper Memory Area (UMA) describes the

reserved 384KB at the top of the first megabyte of system memory.

The way the 384KB of upper memory is used breaks down as follows:The first 128KB after conventional memory is called video RAM and is reserved for use by video adapters.The next 128KB is reserved for the adapter BIOS that resides in read only memory chips on more adapter boards plugged into the bus slots.The last 128KB of memory is reserved for motherboard BIOS.

Extended MemoryExtended memory is basically all memory past the

first megabyte, which can be accessed only while the processor is in protected mode.

Total Installed Memory VS Total Useable Memory

Most people don’t realize that not all the RAM you purchase and install in a system is available.

Because of some quirks in system design, the system usually has to “throw away” up to 384KB of RAM to make way for the Upper Memory Area.

For example, most systems with 16MB of RAM (which is 16,384KB) installed show a total of only 16,000KB installed during the POST or when running Setup.

This usually occurs because the first 384KB of any RAM is reserved from each SIMM. Therefore only the first 384KB of the total RAM can be used, the other 384KB of the second SIMM is “thrown away”